Much has been made about the impact of pitching in the Tigers turn around this year. After allowing 5.3 runs per game in 2008, they have given up just 4.4 runs per game this year, a decrease of almost a full run per game. Obviously, the pitching is substantially better this year with the additions of Edwin Jackson and Rick Porcello and the improvements of Justin Verlander and, to a lesser extent, the bullpen.
While the pitching has been great, the defense is also playing a major role in the success this year. According to the Ultimate Zone Rating statistic at Fan Graphs, the Tigers have improved at every position this year and dramatically so at several positions. Overall, their team defense was 39.1 runs below average in 2008. This year, they are 30.6 runs above average, a turn around of almost 70 runs if they keep it up all season. That's an estimated 7 wins.
Now, let's look at the pitching. In 2008, the Tigers had a FIP (Fielding Independent Pitching) of 4.91. FIP is the the ERA that a pitching staff should have based on things which don't involve fielding - walks, homers, strikeouts and hit batters. In 2009, the Tigers FIP is 4.37, an improvement of 0.54 runs per game. If they keep that up for the entire season, it would be 87 runs. That comes out to an estimated 9 wins.
So, based on UZR and FIP, the Tigers fielding is responsible for about 45% of their improvement in run prevention while pitching accounts for 55%. Of course, you have to be careful when you separate pitching and fielding because they are so intertwined but the two most independent statistics available say that the defense is making a substantial contribution this year. It's doubtful that the pitching staff would look nearly as good this year without the glove work behind them.